Injection molding presses for injecting molding materials into molds are well known in the manufacturing industry. Molding materials such as rubber composites, must typically be cured within their molds under specific temperature and pressure conditions. Prior to the injection of molding material into the mold, the injection press clamps the mold, such that the mold cavity is under pressure during the injection operation. After the injection of the molding material into the mold, the injection press continues to apply the necessary pressure, and perhaps temperature, to cure the molding material.
One disadvantage of conventional molding presses is that they must provide a clamping force which resists both the operating pressure externally applied to the mold during the injection of material into the mold, and a separation force internally applied to the mold as a result of the injection of material and initial curing of the material within the mold. The component of the separation force attributed to the injection of material is proportional to the cross-sectional area of the molding cavity. The clamping or resistance force required must be greater than the operating and separation forces. Thus, as the size of the product to be molded increases, the clamping force required to be applied also increases. Available injection presses provide resistance forces from approximately twenty-five tons, increasing to the necessary resistance force required for the particular application. Obviously, presses capable of applying such forces are quite substantial in size, weight and expense, as they must include support structure to maintain the mold during injection and any initial curing.
An additional disadvantage, is that the time required for each molding cycle may be six minutes or more. A molding cycle typically includes the steps of clamping the mold, injecting material into the mold, holding the mold in clamped condition to initiate curing, and recharging the injection unit with material. Such molding cycle times are required, since the molds must typically remain under pressure within the press to complete the injection operation, initiate curing, and recharge the injection unit with molding material for the next cycle.
Although some systems have attempted to reduce the length of time the molds remain in the press, these systems still require the application of significant pressure by the presses, and the time of each cycle is still relatively long. U.S. Pat. No. 3,973,891, for example, provides an injection press wherein the injection process is performed within ten seconds, and a mold tightening element is then used to maintain the mold in tightened condition during the remainder of the curing process. Once the tightening element is in place, the mold is removed from the injection press and provided to a curing station. Although the mold is not required to be maintained within the injection press for curing after injection, the injection process alone may require a period of ten seconds. The recharging process during which the injector is recharged for the next injection, further increases the total time of the molding cycle.